1. Field of the Invention
The invention is directed to an improved device for variable control of the gas exchange valves of an internal combustion engine, of which at least one gas exchange valve is triggered by a cam of a camshaft, which cam has a cam contour with a circular portion of the cam bottom and a cam apex portion.
2. Description of the Prior Art
The prior art discloses various attempts at realizing a variable control of gas exchange valves. On the one hand, partly variable valve control devices are known in which the valve strokes are variable in stages. One such valve control device is described for instance in European Patent Disclosure EP 0 515 520 B1, which has a tappet made up of two concentric cup elements, of which the inner one rests with one face end on the valve shaft of the gas exchange valve. The tappet cooperates with the cam of a camshaft that has three partial cams with different cam paths. The two outer cam paths have the same stroke course and act on the outer cup element. The middle partial cam has a stroke course deviating from that, with a lesser stroke height, and acts on the inner cup element. The two concentric cup elements can be coupled to one another by hydraulic action of a coupling element, or in a second switching position of this coupling element, they can be moved independently of one another. In the coupled position, the two cup elements are connected to one another, so that they follow the stroke course of the partial cams with a longer stroke. Via the coupling element and the inner cup element, this motion is transmitted to the valve shaft. In the second switching position of the coupling element, the two cup elements are movable independently of one another. The valve shaft cooperates in this switching position with the middle partial cam with a shorter stroke. The outer cup element follows the stroke motion of the outer partial cam, but there is no connection with the inner cup element or the valve shaft. An advantage of such valve drives is that even if the switching mechanism fails, proper valve control is still assured by the camshaft; moreover, conventional internal combustion engines can be equipped without overly great effort. A disadvantage, however, is that the valve strokes and control times are variable in only a limited number of stages, and the staged transition creates problems in terms of noise and operating smoothness. Moreover, such systems are mechanically complicated.
Also from the prior art, such as German Patent Disclosure DE 39 35 218 A1, fully variable valve control devices in the form of electrohydraulic valve drives without a camshaft are known, in which the valve shaft is coupled via a piston rod to a hydraulically actuatable differential piston, by way of which the individual gas exchange valve is actuatable directly, independently of the other gas exchange valves. Consequently, continuously variable valve strokes and variable control times can be achieved for each gas exchange valve. For actuating the differential piston, two control valves per gas exchange valve are used, so that in the four-valve engines that are common at present, eight such control valves per cylinder are needed; because of the high number of components, this has an adverse effect both on the installation space required and on the production costs. Moreover, conventional internal combustion engines can be retrofitted with the new technology only at relatively great effort. Emergency operation properties represent a particular problem, since as a rule, failure of the hydraulic system also causes a total failure of the valve control.